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riginal Article

nalysis of using antirotational device on cephalomedullaryail for proximal femoral fractures�,��

arcelo Itiro Takano, Ramon Candeloro Pedroso de Moraes ∗,uis Gustavo Morato Pinto de Almeida, Roberto Dantas Queiroz

ospital do Servidor Público Estadual de São Paulo, São Paulo, SP, Brazil

r t i c l e i n f o

rticle history:

eceived 9 May 2013

ccepted 16 May 2013

eywords:

ip fractures

racture fixation, internal

one nails

a b s t r a c t

Objective: To analyze the influence of femoral neck diameter in the positioning of the sliding

screw in cefalomedulares nails for treatment of unstable transtrochanteric fractures.

Methods: Prospectively throughout 2011, patients with unstable fractures transtrochanteric

undergoing osteosynthesis with cephalomedullary nail using antirotacional device. They

were evaluated for sex, age and fracture classification according to Tronzo. Through digital

radiographs angle reduction, tip apex distance (TAD), stem diameter and measures between

the positioning of the screws and the limits of the cervix were measured.

Results: Of the 58 patients, 42 (72.4%) were female and 16 (27.6%) were male. 33 patients

were classified as Tronzo III (56.9%), 6 patients as Tronzo IV (10.4%) and 19 as Tronzo V

(19.8%). The majority were in between the eighth and ninth decade of life. The average

reduction in the angle was 130.05◦ for females and 129.4◦ for males. The TAD average was

19.7 mm for females and 21.6 for males. The average diameter of the neck and head vary

with statistical significance between men and women. In 19 patients the placement of the

sliding bolt can be optimal. If the ideal positioning was not possible, the mean displacement

for non-infringement of higher cortical neck was 4.06 mm.

Conclusion: The optimal placement would not be possible for the majority of the population,

for the average diameter of the neck of the sample.

© 2014 Sociedade Brasileira de Ortopedia e Traumatologia. Published by Elsevier Editora

Ltda. All rights reserved.

Análise do emprego do parafuso antirrotacional nos dispositivoscefalomedulares nas fraturas do fêmur proximal

r e s u m o

alavras-chave:

raturas do quadril

ixacão interna de fraturas

inos ortopédicos

Objetivo: analisar a influência do dispositivo antirrotacional no posicionamento do parafuso

deslizante das hastes cefalomedulares usadas no tratamento das fraturas transtrocanteri-

anas.

� Please cite this article as: Takano MI, de Moraes RCP, de Almeida LGMP, Queiroz RD. Análise do emprego do parafuso antirrotacionalos dispositivos cefalomedulares nas fraturas do fêmur proximal. Rev Bras Ortop. 2014;49:17–24.� Study conducted at Hip Group, Department of Orthopedics and Traumatology, Hospital do Servidor Público Estadual de São Paulo, SP,razil.∗ Corresponding author.

E-mail: ramoncpm@yahoo.com.br (R.C.P. de Moraes).255-4971/$ – see front matter © 2014 Sociedade Brasileira de Ortopedia e Traumatologia. Published by Elsevier Editora Ltda. All rights reserved.ttp://dx.doi.org/10.1016/j.rboe.2014.01.001

18 r e v b r a s o r t o p . 2 0 1 4;4 9(1):17–24

Métodos: estudo prospectivo de série de casos composta por 58 pacientes com diagnóstico de

fraturas transtrocanterianas instáveis submetidos à osteossíntese com haste cefalomedu-

lar dotada de dispositivo antirrotacional. A casuística foi avaliada quanto a sexo, idade e

classificacão da fratura. Os parâmetros radiográficos avaliados no pós-operatório imediato

foram: ângulo de reducão, limites anatômicos, distância “ponta-ápice” (TAD), deslocamento

do parafuso deslizante em relacão ao eixo central do colo femoral e posicionamento do

dispositivo antirrotacional.

Resultados: houve preponderância do sexo feminino, com maioria na oitava e nona décadas

de vida. Foram classificados como Tronzo III 33 pacientes (56,9%), seis como Tronzo IV (10,4%)

e 19 como Tronzo V (19,8%). O ângulo de reducão médio no sexo feminino foi 130,5◦ e 129,4◦

no masculino. O diâmetro médio do colo e da cabeca variou com significância estatística

entre homens e mulheres. O TAD médio foi de 19,7 mm no sexo feminino e 21,6 mm no

masculino. Em 10 pacientes (17,85%) o TAD foi superior a 25 mm. Em 19 pacientes (33,9%) a

colocacão do parafuso deslizante poderia ocorrer no eixo central do colo. O deslocamento

médio do implante para não violacão da cortical superior do colo foi de 4,06 mm do eixo

central.

Conclusão: no implante estudado, dotado de dispositivo antirrotacional, o posicionamento

do parafuso deslizante no eixo central do colo está condicionado a diâmetro mínimo de

34 mm do colo femoral.© 2014 Sociedade Brasileira de Ortopedia e Traumatologia. Publicado por Elsevier

Editora Ltda. Todos os direitos reservados.

Introduction

The transtrochanteric fractures correspond to extracapsularfractures of the proximal femur included between the greaterand lesser trochanters.1,2 Of the annual 250,000 proximalfemoral fractures in the U.S.A., 25% are transtrochanteric.3,4

Each year, in developed countries this lesion affects one inevery 1000 people. It is estimated that in 2050 the incidencewill be three times higher3,5 and the annual cost of US$ 8 bil-lion will be duplicated.3,6 Thus, worldwide these fractures areconsidered as a major public health problem.1,2

These are the most frequent fractures, with higher associ-ated mortality rate (12–41% in the first six months),7 and 90%of them, arising from low-energy trauma, occur in patientsolder than 65 years.8

Usually, the treatment is surgical. Only exceptionally theprocedure will be conservative in patients with comorbiditiesthat contraindicate anesthesia, surgery, or both.1,8,9 It is essen-tial that the stability of the fracture be determined, so thatthe surgeon can properly choose the method to be employed.Unstable fractures are those lesions involving the posterior-medial cortex and that feature reverse trace or subtrochantericextension.1,8 Recently, the critical importance of the lateralcortex in regional stability was recognized.10–13

In stable fractures, the implant of choice is the slid-ing hip screw (DHS); however, because of the biomechanicaladvantages of intramedullary location, cephalomedullaryimplants have been advocated for the treatment of unstablefractures.1,14–17

Both for DHS and for cephalomedullary pins, placingthe sliding screw in the correct position is crucial to thesuccess of osteosynthesis. The method of Baumgartner corre-

sponds to the parameter of good positioning currently moreaccepted.1 Anatomical characteristics of certain populationsand factors related to the experience of the surgeon were

related to a placement not always considered “ideal” for theseimplants.18–20

In the evolution process of cephalomedullary pins, theanti-rotational device evolved in order to provide additionalstability to the system, both at the time of its implementa-tion and in maintaining the reduction until the consolidation.However, the presence of an anti-rotational device is relatedto early complications arising from its position, and later, likeas a “Z effect.”1,21

The present study aimed to analyze the influence of the useof anti-rotational device in cephalomedullary pins used in ourinstitution in the average displacement of the sliding screwin its positioning along the central axis of the femoral neck.Furthermore, our study aims to determine the percentageof patients whose tip-apex distance was beyond the recom-mended measure, and the relation of minimum diameter offemoral neck for implant positioning.

Materials and methods

The study was submitted to and approved by the Ethics andResearch Committee of Hospital do Servidor Público Estadualde São Paulo (HSPE). All patients signed an informed consentto participate.

From January to December 2011, a case series comprisedof 58 patients admitted to the emergency room of the HSPEwith preoperative radiographic diagnosis of unstable proximalfemur fracture was prospectively analyzed. The participantswere evaluated for age, gender, and fracture classification.According to the classification of Tronzo,13 fractures type III,variant III, IV and V (Fig. 1) were considered unstable. In theosteosynthesis, we employed the principle of relative stability,

with the cephalomedullary tutor used in our institution. Thesurgical technique was common to all patients and consistedof an indirect reduction of the fracture and osteosynthesis in

r e v b r a s o r t o p . 2 0 1

Type I

Type III

Type IV Type V

Type IIIvariant

Type II

Fig. 1 – Tronzo’s classification.

ofl

aaiastr

i

on the central axis of the neck, we examined the feasibility ofthis positioning in our sample with the analysis of the mini-

rthopedic table by the closed focus technique, with the aid ofuoroscopy.

We used pins with a distal diameter of 10 or 12 mm and with single proximal diameter of 17 mm, mediolateral angle of 6◦

nd cervicodiaphyseal angle of 130◦ between the neck and thentramedullary nail screws. The choice of implant was takenfter preoperative planning, according to the cervicodiaphy-eal angle of the proximal end of the contralateral femur andhe diameter of the medullary diaphyseal region. All patientseceived antithrombotic and antibiotic prophylaxis.

Two cases were excluded because, during the operation,t was decided not to use the anti-rotational device. These

Fig. 2 – Standard positioning f

4;4 9(1):17–24 19

patients were considered only in the assessment of gender,age and Tronzo’s classification.

In the immediate post-operative period, plain digital radio-graphies (anteroposterior [AP] and profile [P] views) of thepelvis and of the hip ipsilateral to the osteosynthesis wereobtained, according to the standardization proposed by Pole-sello et al.22 In AP view, the patient was positioned supinewith the legs in internal rotation of 15–20◦ and with the beamof X-rays directed at the midline just above the pubic symphy-sis. In Acelin’s profile view, the patient was positioned supinewith 90◦ of flexion of the contralateral hip, with the X-ray tubeangled at 45◦ cranially in the horizontal plane, toward the rootof the affected thigh (Fig. 2).

With the use of filing and transmission system Impax®

(version 6.3.1.7501, AGFA Health Care NV), the measures (inmillimeters) of the diameter of the femoral head at its great-est axis, diameter of the neck in its smaller thickness (AB),angle of reduction, distance between the center of the slidingscrew and the top edge of the anti-rotational device (ZX), anddistance from the center of the sliding screw to the inferiormargin of the neck (XB) were digitally obtained in the AP posi-tion. The central axis of the neck was determined using themidpoint of the smaller thickness of the femoral neck (AB).

The distance from the tip of the screw to the apex of thefemoral head was assessed in AP and P (Tip Apex Distance,TAD) views, according to Baumgartner and Solbert method.Fig. 3 shows schematically the points of reference used forthese measurements, and Fig. 4 demonstrates the use ofdigital tools of the Impax® program for obtaining the afore-mentioned measures.

The value of ZX (15 mm) is constant and supplied by themanufacturer. Such information was confirmed in an implantsample with the use of a universal caliper (Fig. 5). To ensurereliability of the data obtained, we applied as individual cor-rection factor for each measurement made the relation of themeasurement of ZX obtained on the digital radiography versusvalue provided by the manufacturer.

Considering as ideal the positioning of the sliding screw

mum neck diameter required and its relation to the size of ZX.Then, we attributed the minimum distance of 2 mm from each

or AP and P radiographs.

20 r e v b r a s o r t o p . 2 0 1 4;4 9(1):17–24

Z

X

B

A

B'

Fig. 3 – Reference points for the proposed measures. AB:diameter of the femoral neck in its smaller thickness. AB′:radius of the femoral neck. X: axis of sliding screw. Z: linetangent to the upper edge of the anti-rotational device. ZX:distance from the axis of the screw to the upper edge of theanti-rotational device.

Fig. 4 – Use of Impax® for the taking of measures. (A) AP radiogrhead. (C) Angle of reduction. (D) TAD in AP. (E) TAD in PF. (F) Dista

Fig. 5 – True measure of distance ZX, with universal caliper.

osseous margin and determined the equation: AB = (ZX + 2) 2.As the value of ZX corresponds to 15 mm, the minimum size ofthe femoral neck for such positioning is 34 mm. Then, we cal-culated the percentage of the sample in which the positioninghere considered as ideal could be obtained.

We measured the average separation of the sliding screwin relation to the central axis of the neck in situations where

an optimum positioning cannot be achieved.

The analyses of the quantitative variables were evalu-ated statistically with respect to the mean, median, standard

aph of the pelvis, as described. (B) Diameter of the neck andnce ZX.

r e v b r a s o r t o p . 2 0 1 4;4 9(1):17–24 21

Table 1 – Age distribution by gender.

Gender Mean Median Standard deviation Minimum Maximum P value

AgeF 81.83 84.00 10.23 48.00 97.00 0.0743M 77.07 77.00 6.23 68.00 90.00

Table 2 – Association between gender and Tronzo’s classification (P = 0.7744).

Tronzo Gender Total

F M N %

N % N %

III 24 57.1 9 56.3 33 56.9IV 5 11.9 1 6.3 6 10.3V 13 31.0 6 37.5 19 32.8

Total 42 100.0 16 100.0 58 100.0

Table 3 – Intraoperative parameters related to the reduction and implant positioning.

Gender Mean Median Standard deviation Minimum Maximum P value

Angle of reduction (AP)F 130.05 132.00 10.42 108.00 158.00 0.8755M 129.40 131.00 9.81 108.00 144.00

TAD2

2

dbtowwTt

R

O1tot

F 19.70 20.10 6.8M 21.67 21.90 6.8

eviation, minimum, and maximum. In the comparisonetween the genders, we applied the nonparametric Wilcoxonest. The qualitative variables were evaluated for distributionf absolute and relative frequencies, and their associationsere tested by Pearson Chi-square test or Fisher exact test,hen the approximation of the first test was not appropriate.he significance level used in these tests was 5%, and optional

wo-tailed hypotheses always were considered.

esults

f the 58 patients in the series, 42 (72.4%) were women and

6 (27.6%) men. In the analysis of the age distribution, most ofhe study patients were between the eighth and ninth decadesf life, and there was no statistically significant difference inhe comparison between genders (Table 1).

Table 4 – Diameter of femoral head and neck obtained accordin

Gender Mean Median Standard deviat

Head (AP)F 49.41 50.00 3.04

M 53.05 54.40 4.45

Neck (AB)F 34.69 34.30 2.99

M 37.23 38.00 4.19

Z X (implant)F 16.75 16.80 1.85

M 15.42 15.50 1.65

1.90 38.50 0.24012.40 30.00

Regarding the classification of the fracture and accordingto Table 2, 33 (56.9%) patients were grouped as Tronzo III, six(10.4%) as Tronzo IV, and 19 (19.8%) as Tronzo V. The associativeanalysis between Tronzo’s classification and gender revealedno statistically significant difference.

With the help of standard X-rays, we obtained dataconcerning the angle of reduction achieved during the intra-operatory period and the implant positioning; no statisticalsignificance was demonstrated when comparing gender,according to Table 3.

Regarding the TAD, it was observed that in 46 cases (82.15%)the values were smaller than 25 mm, thus being consideredideal.

A comparison of neck and head diameter values of theselected patients showed statistically significant differencesbetween genders, which also occurred for the measure of Z X(Table 4).

g to gender.

ion Minimum Maximum P value

43.80 55.10 0.008443.60 58.80

28.90 41.80 0.008626.70 43.10

11.90 22.00 0.019211.60 17.70

22 r e v b r a s o r t o p . 2 0 1 4;4 9(1):17–24

Table 5 – Spreadsheet with values obtained after applying the individual correction of measurement factor.

Nr. Age Gender AB � reduction AB′ Z X Correction factor TAD Tronzo Neck

1 85 F 42 139 20.9 22 1.466666667 17.9 V 28.52 78 F 33 121 16.5 20.5 1.366666667 20.1 III 24.073 74 F 31 147 15.3 20.6 1.373333333 13.3 III 22.284 48 F 35 158 17.5 17.1 1.14 12.6 III 30.615 89 F 36 141 18 15.8 1.053333333 20.2 III 34.086 97 F 35 140 17.7 15.3 1.02 18.6 III 34.77 70 F 33 141 16.6 17.2 1.146666667 20.3 V 23.098 77 M 43 142 21.6 15.2 1.013333333 19.7 V 42.539 79 F 35 136 17.7 16.1 1.073333333 19.7 V 31.21

10 89 F 33 131 16.7 15.4 1.026666667 18.6 V 32.5311 78 M 35 144 17.3 12.8 0.853333333 16.2 V 40.5412 94 F 34 117 16.8 17.5 1.166666667 12.2 III 28.7113 92 F 40 115 20 16 1.066666667 11 III 37.514 90 M 40 131 19.8 16.5 1.1 24.8 V 35.915 80 F 41 135 20.5 18 1.2 23 V 34.1616 85 F 36 115 17.9 20.8 1.386666667 22.5 III 25.7417 92 F 37 136 18.6 17.5 1.166666667 17.6 IV 31.8118 79 F 37 140 18.7 16.7 1.113333333 22 IV 33.5919 92 F 32 108 15.8 16.8 1.12 22 III 28.1220 74 F 35 125 17.5 18 1.2 26 V 29.1621 93 F 39 133 19.4 11.9 0.793333333 21 III 27.5422 87 F 30 125 15.1 15.3 1.02 29.1 III 3023 73 F 30 110 15.2 0 28.3 IV Excluded24 63 F 29 122 14.5 17.8 1.186666667 15.4 III 24.3525 69 M 41 123 20.7 17 1.133333333 26.3 IV 36.5226 79 M 42 119 20.8 15.1 1.006666667 30 III 41.3227 80 F 39 143 19.3 15.4 1.026666667 20.1 III 37.5928 86 F 34 130 17.2 17.4 1.16 25.3 IV 29.6529 75 M 38 127 19 15.5 1.033333333 24.7 III 36.6730 90 F 38 126 18.8 17.4 1.16 29.3 III 32.4131 64 F 33 132 16.3 13.9 0.926666667 16.3 III 34.7432 92 F 37 119 18.3 17.2 1.146666667 29 III 31.8333 84 F 33 139 16.7 16 1.066666667 15.3 III 31.3134 91 F 31 113 15.3 16.3 1.086666667 19.5 III 28.0635 77 F 30 135 14.8 15 1 14.1 V 29.636 87 F 33 121 16.5 15.5 1.033333333 17 III 31.9337 77 M 38 120 19.2 17 1.133333333 20.1 V 33.7938 86 F 36 122 17.8 15.2 1.013333333 23.2 V 35.0339 66 F 33 137 16.5 14.9 0.993333333 30 IV 33.1240 72 M 37 128 18.6 11.6 0.773333333 21.9 III 47.9741 76 M 39 134 19.5 17.7 1.18 2.4 III 33.0542 74 F 34 135 16.9 16.5 1.1 24.6 V 30.7243 70 M 39 134 19.7 14.7 0.98 28.4 III 40.144 95 F 34 123 17.2 17.5 1.166666667 1.9 V 29.445 90 F 34 127 16.9 16.8 1.12 16.9 III 30.0846 81 F 33 116 16.7 16.9 1.126666667 23.2 III 29.6447 68 M 35 134 17.7 14.6 0.973333333 21 III 36.2648 73 F 37 125 18.7 16.9 1.126666667 13 V 33.1949 82 M 31 132 15.7 15.8 1.053333333 17.5 V 28.7150 78 F 33 133 16.5 16.1 1.073333333 38.5 V 30.7451 90 M 35 124 17.4 0 20.1 III Excluded52 75 M 27 108 13.4 17.1 1.14 28.9 V 23.4253 85 F 32 132 16 15.3 1.02 20.3 III 31.3754 87 M 35 123 17.7 16.1 1.073333333 18.6 III 32.8855 84 F 37 125 18.4 16.8 1.12 24 III 32.7656 72 F 37 136 18.7 15.8 1.053333333 20 III 35.5

57 81 M 38 142 19

58 80 F 34 138 16.8

Applying the correction factor for each individual measure-

ment, values taken as real were obtained. These values areshown in Table 5.

Given the corrected diameter of the femoral neck,it was found that an ideal positioning would be

14.6 0.973333333 24.5 III 39.0417.8 1.186666667 32 V 28.23

possible in 19 patients with CI (95%) = 21.8%; 47.8%(Table 6).

For those cases in which the position of the sliding screw

could not be ideal, the mean inferior separation value (XB)found with relation to the central axis was 4.06 (Table 7).

r e v b r a s o r t o p . 2 0 1

Table 6 – Percentage of patients whose placement of thesystem could be ideal.

Ideal possible? Nr. % % valid

Not 37 63.8 66.1Yes 19 32.8 33.9Total 56 96.6 100.0

D

TuttaaDgcb

ppsst

Vihaont

ov

daat

cscesvt

Excluded 2 3.4Total 58 100.0

iscussion

he treatment of unstable transtrochanteric fractures withse of cephalomedullary pins presents biomechanical advan-ages due to its intramedullary location, such as reducinghe bending moment, better rotational control, shortening,nd collapse in varus.1,14–17 Although controversial, therere reports of superiority of cephalomedullary pins versusHS in relation to early return to ambulation, reduced sur-ical time, and less blood loss.1,8,19 Thus, at our institutionephalomedullary pins are applied in the treatment of unsta-le fractures.1

Regarding the epidemiological findings observed in theresent study, there is a vast literature attesting the femaleredominance and an age close to 80 years in othereries.1,9,18,21,23 This reflects the decrease in bone mineral den-ity. We found no statistically significant difference betweenhe ages of male and female patients.

Although the pattern fracture of reverse obliquity (Tronzo or AO 31 A3) has been reported as the most frequent type

n some case series of unstable fractures,9,24 most studiesave described as the most prevalent types those classifieds Tronzo III or IV (or AO 31 A2),1,18,25,26 which agrees withur results. When comparing males and females, we foundo statistically significant differences in relation to fractureype according to the classification of Tronzo.

According to Werner-Tutschku et al.,27 the main predictorf the cutout is unsatisfactory initial reduction, especially inarus, besides favoring Trendelenburg gait.

We found a mean value of 130.5◦ for men (with a standardeviation of 10.42) and 129.40◦ (9.81) for women, and thisssessment was not statistically significant. These findingsre similar to those found in another study with unstableranstrochanteric fractures.1

There was a statistically significant difference in theomparison between male and female genders in the mea-urements related to diameter of the head and neck. Whenomparing the bone structure of the neck in young and

28

lderly subjects of Chinese and Caucasian origin, Wang et al.howed that male subjects have larger diameters, that thisalue tends to increase with age, being higher in popula-ions of white origin. According to Pu et al.,18 as the Chinese

Table 7 – Displacement of the system in relation to the central a

Mean Standard deviation CI (95%)

Inf. thres. Sup

Neck corrected 4.06 2.95 3.07 5

4;4 9(1):17–24 23

population is of lower stature than that of the European sub-jects, the length of the proximal femur and the diameter ofthe femoral neck are also smaller, which leads to the inappro-priate positioning of the cephalomedullary pin’s spiral bladeused in the study, or to redundancy of the proximal end of thepin. In the utilization of the cephalomedullary pin used in ourservice, the minimum diameter for an optimal placement is34 mm, which corresponds to twice the Z X measure, takinginto account a thickness of cortical (top and bottom) of 4 mm.In our series, in only 19 patients (32.8%) the placement of thesliding screw could be performed in the situation regarded asideal by our methodology. Extrapolating the confidence inter-val for the Brazilian population, only in 21.8–47.8% of patientsin 95% of the time the implant could be placed optimally (i.e.,the center of the sliding screw located along the central axisof the neck).

According to Baumgaertner et al.,15 the correct implantplacement occurs when the distance between the tip of thesliding screw and the femoral head center does not exceed25 mm after the sum of the values obtained on anteroposteriorand profile views (tip-apex index, or TAD <25 mm). This facili-tates the telescoping of the dynamic system of the implant andreduces the risk of cutout.9 Although described for osteosyn-thesis with DHS, the method can be used to assess the correctpositioning of the cephalomedullary pins.1 However, in pinswith two proximal fixation screws, there is difficulty in thepositioning of the sliding screw in the center of the femoralhead in the anteroposterior view. Thus, there is a greater ten-dency for the positioning of the sliding screw in a more inferiorlocation in the AP radiographic view, particularly in patientswith short femoral neck and head.21 The location of the screwsin the profile position is not affected, since the screws are par-allel. In the osteosynthesis using the cephalomedullary pinin question, the mean displacement necessary for the intro-duction of the nail without violating the anti-rotational uppercortical neck was of 4.6 mm (i.e., the amount of downwarddisplacement of the system, in millimeters, from the axisof the femoral neck). To calculate the required displacementof the sliding screw relative the central axis of the neck insituations where this option is not possible, the following for-mula: = displacement required = 34 mm − neck size (AB) wasapplied.

Conclusion

Considering the mean diameter of the neck in our sample,

the positioning on the central axis of the neck would not bepossible for the majority of the population.

Considering the implant studied, the minimum size of theneck which allows positioning the central axis is 34 mm.

xis.

Median Q25 Q75 Minimum Maximum

. thres.

.04 3.91 1.59 5.29 0.21 11.72

p . 2 0

r

1

1

1

1

1

1

1

1

1

1

2

2

2

2

2

2

2

2

means of PFN. Unfallchirurg. 2002;105(10):881–5.

24 r e v b r a s o r t o

In situations where the positioning in the central axis isnot possible due to the minimal size of the neck, the down-ward displacement required can be calculated by the formula:displacement required = 34 − neck size (AB).

Conflicts of interest

The authors declare no conflicts of interest.

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